Field of the Invention
The present invention relates to methods for the manufacture of composite optical materials and to composite materials obtainable by such methods. The invention has particular, but not exclusive, applicability to materials which display structural colour characteristics.
Related Art
Natural opal is built up from domains consisting of monodisperse silica spheres of diameter 150-400 nm. These spheres are close-packed and therefore form a regular three dimensional lattice structure within each domain. The colour play of such opals is created by Bragg-like scattering of the incident light at the lattice planes of the domains.
It is known to produce synthetic opal-like materials. For example, U.S. Pat. No. 4,703,020 discloses the formation of such materials by allowing silica spheres to sediment from an aqueous dispersion. This sediment is then dried and calcined at 800° C. Subsequently, a solution of zirconium alkoxide is allowed to penetrate into the interstices in the sediment and zirconium oxide is precipitated in the interstices by hydrolysis. The material is then calcined again to leave a structure in which silica spheres are arranged in a three dimensional lattice with zirconium oxide in the interstices. Forming opal-like materials in this way is exceptionally time-consuming and expensive. It is not an industrially-applicable route for the manufacture of significant quantities of materials.
US 2004/0253443 (equivalent to WO03025035) discloses moulded bodies formed from core-shell particles. Each particle is formed of a solid core, and the solid cores have a monodisperse particle size distribution. Each particle has a shell formed surrounding the core. The core and shell have different refractive indices. In one embodiment in this document, the core is formed of crosslinked polystyrene and the shell is formed of a polyacrylate such as polymethyl methacrylate (PMMA). In this case, the core has a relatively high refractive index and the shell has a relatively low refractive index. A polymer interlayer may be provided between the core and shell, in order to adhere the shell to the core. Granules of the core-shell particles are heated and pressed to give a film. In this heating and pressing step shell material is soft but the core material remains solid. The cores form a three dimensional periodic lattice arrangement, and the shell material becomes a matrix material. The resultant material demonstrates an optical opalescent effect. Inorganic nanoparticles (e.g. metal nanoparticles or semiconductor nanoparticles) can be incorporated in the interstices between cores to provide enhanced functionality to the material. US 2004/0253443 suggests mechanisms to explain the ordering of the core particles in the matrix, but these are not fully explained.
US 2005/0142343 (equivalent to WO03064062) provides similar disclosure to US 2004/0253443. However, additionally, a contrast material such as a pigment is stored in the matrix, in order to enhance the optical effect.
US 2005/0228072 (equivalent to WO03106557) provides similar disclosure to US 2004/0253443. However, additionally, a further material is added in order to control the mechanical properties of the composite material. The further material is, for example, a thermoplastic rubber polymer.
WO2004096894 provides similar disclosure to US 2004/0253443, and additionally proposes extruding the composite material as a sheet and subsequently rolling the material. The result is reported to be a uniform colour effect depending on the viewing angle.
Pursiainen et al [O. Pursiainen, J. Baumberg, H. Winkler, B. Viel, P. Spahn and T. Ruhl “Nanoparticle-tuned structural color from polymer opals” Optics Express, 23 Jul. 2007, Vol. 15, No. 15, 9553] discuss the effect of the incorporation of nanoparticles on the optical properties of flexible polymer opals formed using core-shell polymeric particles. The core of the core-shell particles is formed from polystyrene and the outer shell is formed from polyethylacrylate. Ordering of the particles is achieved by causing flow of the particles between two outer covering layers under compression.